Bioactive peptides are small peptides that elicit a biological activity. Since the discovery of secretin in 1902, over 500 of these peptides which average 20 amino acids in size have been identified and characterized. They have been isolated from a variety of systems, exhibit a wide range of actions, and have been utilized as therapeutic agents in the field of medicine and as diagnostic tools in both basic and applied research. Tables 1 and 2 list some of the best known bioactive peptides.
Where the mode of action of these peptides has been determined, it has been found to be due to the interaction of the bioactive peptide with a specific protein target. In most of the cases, the bioactive peptide acts by binding to and inactivating its protein target with extremely high specificities. Binding constants of these peptides for their protein targets typically have been determined to be in the nanomolar (nM, 10xe2x88x929 M) range with binding constants as high as 10xe2x88x9212 M (picomolar range) having been reported. Table 3 shows target proteins inactivated by several different bioactive peptides as well as the binding constants associated with binding thereto.
Recently, there has been an increasing interest in employing synthetically derived bioactive peptides as novel pharmaceutical agents due to the impressive ability of the naturally occurring peptides to bind to and inhibit specific protein targets. Synthetically derived peptides could be useful in the development of new antibacterial, antiviral, and anticancer agents. Examples of synthetically derived antibacterial or antiviral peptide agents would be those capable of binding to and preventing bacterial or viral surface proteins from interacting with their host cell receptors, or preventing the action of specific toxin or protease proteins. Examples of anticancer agents would include synthetically derived peptides that could bind to and prevent the action of specific oncogenic proteins.
To date, novel bioactive peptides have been engineered through the use of two different in vitro approaches. The first approach produces candidate peptides by chemically synthesizing a randomized library of 6-10 amino acid peptides (J. Eichler et al., Med. Res. Rev. 15:481-496 (1995); K. Lam, Anticancer Drug Des. 12:145-167 (1996); M. Lebl et al., Methods Enzymol. 289:336-392 (1997)). In the second approach, candidate peptides are synthesized by cloning a randomized oligonucleotide library into a Ff filamentous phage gene, which allows peptides that are much larger in size to be expressed on the surface of the bacteriophage (H. Lowman, Ann. Rev. Biophys. Biomol. Struct. 26:401-424 (1997); G. Smith et al., et al. Meth. Enz. 217:228-257 (1993)). To date, randomized peptide libraries up to 38 amino acids in length have been made, and longer peptides are likely achievable using this system. The peptide libraries that are produced using either of these strategies are then typically mixed with a preselected matrix-bound protein target. Peptides that bind are eluted, and their sequences are determined. From this information new peptides are synthesized and their inhibitory properties are determined. This is a tedious process that only screens for one biological activity at a time.
Although these in vitro approaches show promise, the use of synthetically derived peptides has not yet become a mainstay in the pharmaceutical industry. The primary obstacle remaining is that of peptide instability within the biological system of interest as evidenced by the unwanted degradation of potential peptide drugs by proteases and/or peptidases in the host cells. There are three major classes of peptidases which can degrade larger peptides: amino and carboxy exopeptidases which act at either the amino or the carboxy terminal end of the peptide, respectively, and endopeptidases which act on an internal portion of the peptide. Aminopeptidases, carboxypeptidases, and endopeptidases have been identified in both prokaryotic and eukaryotic cells. Many of those that have been extensively characterized were found to function similarly in both cell types. Interestingly, in both prokaryotic and eukaryotic systems, many more arninopeptidases than carboxypeptidases have been identified to date.
Approaches used to address the problem of peptide degradation have included the use of D-amino acids or modified amino acids as opposed to the naturally occurring L-amino acids (e.g., J. Eichler et al., Med Res Rev. 15:481-496 (1995); L. Sanders, Eur. J. Drug Metabol. Pharmacokinetics 15: 95-102 (1990)), the use of cyclized peptides (e.g., R. Egleton, et al., Peptides 18: 1431-1439 (1997)), and the development of enhanced delivery systems that prevent degradation of a peptide before it reaches its target in a patient (e.g., L. Wearley, Crit. Rev. Ther. Drug Carrier Syst. 8: 331-394 (1991); L. Sanders, Eur. J. Drug Metabol. Pharmacokinetics 15: 95-102 (1990)). Although these approaches for stabilizing peptides and thereby preventing their unwanted degradation in the biosystem of choice (e.g., a patient) are promising, there remains no way to routinely and reliably stabilize peptide drugs and drug candidates. Moreover, many of the existing stabilization and delivery methods cannot be directly utilized in the screening and development of novel useful bioactive peptides. A biological approach that would serve as both a method of stabilizing peptides and a method for identifying novel bioactive peptides would represent a much needed advance in the field of peptide drug development.
The present invention provides an intracellular screening method for identifying novel bioactive peptides. A host cell is transformed with an expression vector comprising a tightly regulable control region operably linked to a nucleic acid sequence encoding a peptide. The transformed host cell is first grown under conditions that repress expression of the peptide and then, subsequently, expression of the peptide is induced. Phenotypic changes in the host cell upon expression of the peptide are indicative of bioactivity, and are evaluated. If, for example, expression of the peptide is accompanied by inhibition of host cell growth, the expressed peptide constitutes a bioactive peptide, in that it functions as an inhibitory peptide.
Intracellular identification of bioactive peptides can be advantageously carried out in a pathogenic microbial host cell. Bioactive peptides having antimicrobial activity are readily identified in a microbial host cell system. Further, the method can be carried out in a host cell that has not been modified to reduce or eliminate the expression of naturally expressed proteases or peptidases. When carried out in a host cell comprising proteases and peptides, the selection process of the invention is biased in favor of bioactive peptides that are protease- and peptidase-resistant.
The tightly regulable control region of the expression vector used to transform the host cell according to the invention is preferably derived from the wild-type Escherichia coli lac operon, and the transformed host preferably comprises an amount of Lac repressor protein effective to repress expression of the peptide during host cell growth under repressed conditions. To insure a sufficient amount of Lac repressor protein, the host cell can be transformed with a second vector that overproduces Lac repressor protein.
Optionally, the expression vector used to transform the host cell can be genetically engineered to encode a stabilized peptide that is resistant to peptidases and proteases. For example, the coding sequence can be designed to encode a stabilizing group at either or both of the peptide""s N-terminus or C-terminus. As another example, the coding sequence can be designed to encode a stabilizing motif such as an xcex1-helix motif or an opposite charge ending motif, as described below. The presence of a stabilizing group at a peptide terminus or a stabilizing motif can slow down the rate of intracellular degradation of the peptide.
The invention further provides a bioactive peptide having a first stabilizing group comprising the N-terminus and a second stabilizing group comprising the C-terminus. Preferably, the first stabilizing group is selected from the group consisting of a small stable protein. Pro-, Pro-Pro-, Xaa-Pro- and Xaa-Pro-Pro-; and the second stabilizing group is selected from the group consisting of a small stable protein, -Pro, -Pro-Pro, -Pro-Xaa and -Pro-Pro-Xaa. Suitable small stable proteins include Rop protein, glutathione sulfotransferase, thioredoxin, maltose binding protein, and glutathione reductase. In addition, the invention provides a bioactive peptide stabilized by an opposite charge ending motif, as described below. The bioactive peptide is preferably an antimicrobial peptide or a therapeutic peptide drug.
Also provided by the invention is a polyeptide that can be cleaved to yield a bioactive peptide having a stabilizing group at either or both of its N- and C-termini. The cleavable polypeptide accordingly comprises a chemical or enzymatic cleavage site either immediately preceding the N-terminus of the bioactive peptide or immediately following the C-terminus of the bioactive peptide.
The invention further provides a fusion protein comprising a four-helix bundle protein, preferably the Rop protein, and a polypeptide. The four-helix bundle protein is positioned at either the N-terminus or the C-terminus of the fusion protein, and accordingly can be fused to either the N-terminus or the C-terminus of the polypeptide.
The present invention also provides a method for using an antimicrobial peptide. An antimicrobial peptide is stabilized by linking a first stabilizing group to the N-terminus of an antimicrobial peptide, and, optionally, a second stabilizing group to the C-terminus of the antimicrobial peptide. Alternatively, the antimicrobial peptide is stabilized by flanking the peptide sequence with an opposite charge ending motif, as described below. The resulting stabilized antimicrobial peptide is brought into contact with a microbe, preferably a pathogenic microbe, for example to inhibit the growth or toxicity of the microbe.
The invention also provides a method for treating a patient having a condition treatable with a peptide drug, comprising administering to the patient a stabilized peptide drug having at least one of a first stabilizing group comprising the N-terminus of the stabilized peptide drug and a second stabilizing groupj comprising the C-terminus of the stabilized peptide drug. Optionally, prior to administration of the stabilized peptide drug, the first stabilizing group is covalently linked to the N-terminus of a peptide drug, and the second stabilizing group is covalently linked to the C-terminus of the peptide administering to the patient a peptide drug that has been stabilized by flanking the peptide sequence with an opposite charge ending motif, as described below.